I understand the material difference in some of the components would lead to less rust on the Ultegra. Further, there may be differences in the quality of the bearing. May or may not be relevant to your needs.

I use the 105 SPD-SL pedals on my road and track bikes, great looking pedals and seem to last well. Prefer SPD-SL to SPD when it comes to long distance and hard riding, SPD is way nicer for commuting though.